Shortly before Election Day last year, some helpful-looking posts began popping up on Twitter: No need to stand in line or even leave home, they said — just vote by text! The messages, some of which appeared to come from Hillary Clinton's campaign, had versions in Spanish, with gay pride flags and other permutations. They were also 100% false. Where did they come from? Twitter took the posts down . But answering the question about who created the misleading posts — whether the Russian influence-mongers who were running a big disinformation campaign in the United States, or opposition dirty tricksters, or someone else — has proven to be vexing for congressional investigators, people familiar with the matter said. "We have not found accounts associated...